U.S. patent number 7,484,473 [Application Number 10/869,563] was granted by the patent office on 2009-02-03 for suspended gas distribution manifold for plasma chamber.
This patent grant is currently assigned to Applied Materials, Inc.. Invention is credited to Ernst Keller, Quanyuan Shang.
United States Patent |
7,484,473 |
Keller , et al. |
February 3, 2009 |
Suspended gas distribution manifold for plasma chamber
Abstract
A gas inlet manifold for a plasma chamber having a perforated
gas distribution plate suspended by a side wall comprising one or
more sheets. The sheets preferably provide flexibility to alleviate
stress in the gas distribution plate due to thermal expansion and
contraction. In another aspect, the side wall provides thermal
isolation between the gas distribution plate and other components
of the chamber.
Inventors: |
Keller; Ernst (Sunnyvale,
CA), Shang; Quanyuan (Saratoga, CA) |
Assignee: |
Applied Materials, Inc. (Santa
Clara, CA)
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Family
ID: |
25446716 |
Appl.
No.: |
10/869,563 |
Filed: |
June 15, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050000432 A1 |
Jan 6, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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09922219 |
Aug 3, 2001 |
6772827 |
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09488612 |
Jan 20, 2000 |
6477980 |
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Current U.S.
Class: |
118/723R;
118/715; 156/345.26; 156/345.33; 156/345.43 |
Current CPC
Class: |
C23C
16/455 (20130101); C23C 16/45565 (20130101); C23C
16/5096 (20130101); H01J 37/3244 (20130101); Y10T
29/49948 (20150115) |
Current International
Class: |
C23C
16/455 (20060101); C23C 16/50 (20060101); C23C
16/06 (20060101); C23C 16/22 (20060101) |
Field of
Search: |
;118/723R,715,715E,715ER
;156/345.29,345.33,345.26,345.43 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Zervigon; Rudy
Attorney, Agent or Firm: Stern; Robert J.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This patent application is a divisional of application Ser. No.
09/922,219 filed Aug. 3, 2001 now U.S. Pat. No. 6,772,827 by Ernst
Keller et al., which is a continuation-in-part of application Ser.
No. 09/488,612 filed Jan. 20, 2000 by John White et al., now U.S.
Pat. No. 6,477,980.
Claims
The invention claimed is:
1. A gas inlet manifold for a plasma chamber, comprising: a top
wall including a gas inlet orifice; a gas distribution plate
including a plurality of gas outlet orifices, wherein the gas
distribution plate is spaced away from the top wall, and a side
wall including one or more side wall segments, wherein each side
wall segment includes an upper portion, a lower flange, and a
vertically oriented sheet extending between the upper portion and
the lower flange; wherein the upper portion of each side wall
segment is connected to the top wall of the gas inlet manifold; and
wherein the gas distribution plate further comprises (i) a lip
extending radially outward from the perimeter of the gas
distribution plate, and (ii) a plurality of pins attached to, and
extending downward from, the lip of the gas distribution plate;
wherein the lower flange of each segment of the side wall includes
a plurality of holes; wherein the lip of the gas distribution plate
rests on each lower flange so that each of said pins extends
through a corresponding one of said holes; and wherein each hole
has a width that exceeds the width of its corresponding pin so as
to permit relative movement between each lower flange and the gas
distribution plate.
2. A gas inlet manifold according to claim 1, wherein: each sheet
is flexible so as to permit movement of the lower flange in a
direction perpendicular to the sheet; and for each segment of the
side wall, each hole in the lower flange of that segment has a long
axis parallel to the sheet of that segment.
3. A gas inlet manifold according to claim 1, wherein: the width of
each hole along one axis of the hole exceeds the width of its
corresponding pin along said axis by an amount sufficient to permit
an amount of relative movement between each lower flange and the
gas distribution plate that exceeds the maximum likely relative
differential thermal expansion between the lower flange and the gas
distribution plate during operation of the plasma chamber.
4. A gas inlet manifold according to claim 1, wherein: the width of
each hole along one axis of the hole exceeds the width of its
corresponding pin along said axis by at least 0.03 inch.
5. A gas inlet manifold according to claim 1, wherein: the width of
each hole along one axis of the hole exceeds the width of its
corresponding pin along said axis by at least 0.1% of the widest
dimension of the gas distribution plate.
6. A gas inlet manifold according to claim 1, wherein the sheet of
each side wall segment is flexible.
Description
FIELD OF THE INVENTION
The invention relates generally to gas distribution manifolds for
supplying gas to a plasma chamber. More specifically, the invention
relates to such a manifold having a perforated gas distribution
plate suspended by a thin side wall.
BACKGROUND OF THE INVENTION
Electronic devices, such as flat panel displays and integrated
circuits, commonly are fabricated by a series of process steps in
which layers are deposited on a substrate and the deposited
material is etched into desired patterns. The process steps
commonly include plasma enhanced chemical vapor deposition (CVD)
processes and plasma etch processes.
Plasma processes require supplying a process gas mixture to a
vacuum chamber called a plasma chamber, and then applying
electrical or electromagnetic power to excite the process gas to a
plasma state. The plasma decomposes the gas mixture into ion
species that perform the desired deposition or etch process.
In capacitively excited CVD chambers, the plasma is excited by RF
power applied between an anode electrode and a cathode electrode.
Generally the substrate is mounted on a pedestal or susceptor that
functions as the cathode electrode, and the anode electrode is
mounted a short distance from, and parallel to, the substrate.
Commonly the anode electrode also functions as a gas distribution
plate for supplying the process gas mixture into the chamber. The
anode electrode is perforated with hundreds or thousands of
orifices through which the process gas mixture flows into the gap
between the anode and cathode. The orifices are spaced across the
surface of the gas distribution plate so as to maximize the spatial
uniformity of the process gas mixture adjacent the substrate. Such
a gas distribution plate, also called a diffuser or "shower head",
is described in commonly assigned U.S. Pat. No. 4,854,263 issued
Aug. 8, 1989 to Chang et al.
Perforated gas distribution plates typically are rigidly mounted to
the lid or upper wall of the plasma chamber. Rigid mounting has the
disadvantage of not accommodating thermal expansion of the
perforated plate as it acquires heat from the plasma. The
consequent mechanical stresses on the plate can distort or crack
the plate. Alleviating mechanical stress is most important with the
larger distribution plates required to process larger workpieces,
such as large flat panel displays. Therefore, a need exists for a
gas distribution device that minimizes such thermally induced
mechanical stresses.
Another shortcoming of conventional diffusers or gas distribution
plates is that they commonly operate at temperatures that are
undesirably low and spatially non-uniform. Specifically, while the
diffuser receives heat from the plasma in the chamber, a
conventional diffuser generally loses heat at its perimeter where
it is bolted to the chamber wall or lid. Therefore, the perimeter
of the diffuser is significantly cooler than the center, which
tends to produce a corresponding undesirable spatial non-uniformity
in the surface temperature of the substrate positioned near the
diffuser. Furthermore, the heat loss from the diffuser to the
chamber wall undesirably reduces the temperature of the diffuser,
which can undesirably reduce the substrate temperature.
SUMMARY OF THE INVENTION
The invention is a gas inlet manifold for a plasma chamber used for
processing a substrate. The manifold has a perforated gas
distribution plate or diffuser suspended by a side wall.
In one aspect of the invention, the side wall of the inlet manifold
comprises one or more sheets. One advantage of suspending the
diffuser by a sheet is that the sheet can be flexible so as to
accommodate thermal expansion or contraction of the gas
distribution plate, thereby avoiding distortion or cracking of the
diffuser. Another advantage is that the sheet can interpose a
substantial thermal impedance between the diffuser and cooler
chamber components so as to improve the spatial uniformity of the
diffuser temperature and reduce heat loss from the substrate to the
diffuser.
In a preferred embodiment, each sheet has a long, narrow flange at
its lower end. Each flange has a plurality of holes along its
length that mate with pins mounted in the rim of the gas
distribution plate. The holes are elongated in a direction parallel
to the long dimension of the flange so as to permit differential
movement between the flexible side wall and the gas distribution
plate.
In another preferred embodiment, the flexible side wall has a
plurality of segments separated by small gaps, and the manifold
includes a novel sealing flange that minimizes gas leakage through
the gaps while permitting movement of the flexible side wall
segments.
In a second aspect of the invention, the inlet manifold side wall
interposes substantial thermal impedance between the gas
distribution plate and the chamber wall, thereby improving the
spatial uniformity of the temperature of the gas distribution
plate, as well as allowing the gas distribution plate to attain a
higher temperature in response to heating from the plasma. This
aspect of the invention helps improve spatial uniformity of the
surface temperature of the substrate or workpiece, and it enables
the workpiece to be reach a higher surface temperature relative to
the temperature of the substrate support pedestal or susceptor. In
this aspect of the invention, the side wall need not comprise a
sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional, partially schematic side view of a plasma
chamber that includes the gas inlet manifold of the present
invention.
FIG. 2 is a partially exploded perspective view of a corner of the
gas inlet manifold.
FIG. 3 is a transverse sectional view of a corner support of the
gas inlet manifold.
FIG. 4 is a vertical sectional view of one side of one embodiment
of the gas inlet manifold in which the side wall is rigidly
attached to the diffuser.
FIG. 5 is a vertical sectional view of one side of a more preferred
gas inlet manifold in which the side wall can slide within a groove
in the diffuser.
FIG. 6 is a plan view of the lower flange of the inlet manifold
side wall having elongated holes to accommodate thermal expansion
of the diffuser.
FIG. 7 is a vertical sectional view of one side of an alternative
gas inlet manifold in which the diffuser has no circumferential
groove.
FIG. 8 is a vertical sectional view of a corner of the gas inlet
manifold.
FIG. 9 is an exploded view of the corner shown in FIG. 2.
FIG. 10 is a plan view of an alternative corner junction or coupler
before it is folded.
FIG. 11 is an exploded view of a corner having the alternative
coupler of FIG. 10.
FIG. 12 is a view similar to FIG. 4 of an alternative embodiment
having a gas inlet manifold in which a portion of the top flange of
the flexible side wall is exposed to atmospheric pressure.
FIG. 13 is a detail of FIG. 12.
FIG. 14 is a view similar to FIG. 2 of the alternative embodiment
of FIG. 12.
FIG. 15 is a view similar to FIG. 13 showing an electrical cable
connected directly to the top flange of the side wall of the gas
inlet manifold.
FIG. 16 is a partially exploded perspective view of a corner of an
alternative gas inlet manifold in which the flexible side walls
abut at the corners and the corner couplers are omitted.
FIG. 17 is a plan view of the lower flange of the inlet manifold
side wall having enlarged holes to accommodate thermal expansion of
the diffuser.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Plasma Chamber Overview
FIG. 1 shows a plasma chamber that includes a gas inlet manifold
20-32, also called a gas distribution manifold or plenum, according
to the present invention. The illustrated chamber is suitable for
performing plasma-assisted processes such as chemical vapor
deposition (CVD) or etching on a large substrate. It is especially
suitable for performing CVD processes for fabricating the
electronic circuitry of a flat panel display on a glass
substrate.
The plasma chamber or vacuum chamber has a housing or wall 10,
preferably composed of aluminum, that encircles the interior of the
chamber. The chamber wall 10 provides the vacuum enclosure for the
side, and much of the bottom, of the chamber interior. A metal
pedestal or susceptor 12 functions as a cathode electrode and has a
flat upper surface that supports a workpiece or substrate 14.
Alternatively, the substrate need not directly contact the
susceptor, but may be held slightly above the upper surface of the
susceptor by, for example, a plurality of lift pins, not shown.
An external gas supply, not shown, delivers one or more process
gases to the process chamber. Specifically, the chamber includes a
gas inlet manifold or plenum 20-32 (described in detail below) that
encloses a region referred to as the manifold interior. A gas line
or conduit extending from the external gas supply to a gas inlet
aperture or orifice 30 in a top wall or back wall 28 of the gas
inlet manifold supplies the process gases into the manifold
interior. The gases then flow out of the manifold through hundreds
or thousands of orifices 22 in a gas distribution plate or diffuser
20 so as to enter the region of the chamber interior between the
gas distribution plate and the susceptor 12.
A conventional vacuum pump, not shown, maintains a desired level of
vacuum within the chamber and exhausts the process gases and
reaction products from the chamber through an annular exhaust slit
42, then into annular exhaust plenum 44, and then through an
exhaust channel, not shown, to the pump.
The gas distribution plate or diffuser 20 is composed of an
electrically conductive material, preferably aluminum, so that it
can function as an anode electrode. An RF power supply, not shown,
is connected between the gas distribution plate and the
electrically grounded chamber components. A typical frequency for
the RF power supply is 13 MHz. Because it is RF hot, the diffuser
or gas distribution plate 20 is electrically insulated from the lid
by annular dielectric spacers 34, 35, 36. The chamber side and
bottom wall 10 and the lid 18 are connected to electrical ground.
The susceptor or workpiece support pedestal 12 typically is
grounded also, but it optionally can be connected to a second RF
power supply, commonly called the bias power supply.
The RF power applied between the cathode electrode (the susceptor
12) and the anode electrode (the gas distribution plate 20)
produces an electromagnetic field in the region between the two
electrodes that excites the gases in that region to a plasma state.
The plasma produces reactive species from the process gas mixture
that react with exposed material on the workpiece to perform the
desired deposition or etch process.
To concentrate the plasma in the region of the chamber between the
workpiece 14 and the gas distribution plate 20, other metal
surfaces in the chamber that are near the distribution plate
preferably are covered with dielectric liners. Specifically, a
dielectric liner 37 is bolted to the underside of the chamber lid
18, and dielectric liner 38 covers the chamber side wall 10. To
prevent plasma formation, and to minimize RF power conduction, in
the annular gap between the gas inlet manifold and the lid, a
dielectric liner 41 occupies that gap.
A removable lid 18 rests atop the chamber side wall 10 so that the
lid functions as an additional portion of the chamber wall. The gas
inlet manifold 20-32 rests on an annular, inwardly extending shelf
of the lid. A cover 16 is clamped to the top of the lid 18. The
only purpose of the cover is to protect human personnel from
accidental contact with the portions of the gas inlet manifold that
are RF hot, as described below.
The chamber components should be composed of materials that will
not contaminate the semiconductor fabrication processes to be
performed in the chamber and that will resist corrosion by the
process gases. Aluminum is our preferred material for all of the
components other than the dielectric spacers and liners 34-41 and
the O-rings 45-48.
All portions of the plasma chamber other than the gas inlet
manifold are conventional. The design and operation of conventional
plasma CVD and etch chambers are described in the following
commonly-assigned U.S. patents, the entire content of each of which
is hereby incorporated by reference in this patent specification:
U.S. Pat. No. 5,844,205 issued Dec. 1, 1998 to White et al.; and
U.S. Pat. No. 4,854,263 issued Aug. 8, 1989 to Chang et al.
Gas Inlet Manifold
FIGS. 2-4 show the gas inlet manifold or plenum in more detail. The
gas inlet manifold has an interior region that is bounded on the
bottom by the gas distribution plate or diffuser 20, on the sides
by the flexible side wall or suspension 24, and on the top by the
top wall or back wall 28. (The triangular corner support post 58
shown in FIGS. 2 and 3 will be described later.)
In the illustrated embodiments, the gas distribution plate 20 is an
aluminum plate that is 3 cm thick. Preferably it should be thick
enough so that it is not significantly deformed under atmospheric
pressure when a vacuum is created within the chamber.
In our novel gas inlet manifold design, the gas distribution plate
20 is suspended by a thin, flexible side wall or suspension 24, so
that the suspension supports the entire weight of the gas
distribution plate. As explained in the section below entitled
"Flexible Suspension to Accommodate Thermal Expansion and
Contraction", the suspension is flexible to minimize stress on the
gas distribution plate in response to its thermal expansion and
contraction. The upper portion of the flexible side wall has an
upper flange 26 that is directly or indirectly mounted to and
supported by the chamber wall 10. By "indirect" mounting and
support, we mean that the upper portion of the suspension may be
supported by the chamber wall through intermediate components that
are interposed between the upper flange 26 and the chamber wall 10,
such as the lid 18 and the inlet manifold back wall 28 in the
embodiment of FIG. 1.
The top wall or back wall 28 of the gas inlet manifold is mounted
so as to abut the upper portion or upper flange 26 of the
suspension, so that the back wall forms the upper boundary or
enclosure of the interior region of the gas inlet manifold.
In the illustrated embodiments having a rectangular diffuser or gas
distribution plate 20, the flexible side wall or suspension 24
preferably consists of four facets or segments, where each segment
is a distinct piece of thin, flexible sheet metal. Each of the four
segments of the side wall is attached to a corresponding one of the
four sides of the gas distribution plate. The four segments or
facets of the side wall or suspension 24 collectively encircle the
interior of the gas inlet manifold.
The orifices 22 in the gas distribution plate should have a
diameter smaller than the width of the plasma dark space in order
to prevent plasma within the plasma chamber from entering the
region enclosed by the gas inlet manifold, i.e., the region between
the gas distribution plate 20 and the top wall or back wall 28 of
the inlet manifold. The width of the dark space, and therefore the
optimum diameter of the orifices, depends on chamber pressure and
other parameters of the specific semiconductor fabrication
processes desired to be performed in the chamber. Alternatively, to
perform plasma processes using reagent gases that are especially
difficult to dissociate, it may be desirable to employ orifices
having a narrow inlet and a wider, flared outlet as described in
the above-referenced U.S. Pat. No. 4,854,263 to Chang et al.
Preferably the gas inlet manifold also includes a gas inlet
deflector consisting of a circular disc 32 having a diameter
slightly greater than that of the gas inlet orifice 30 and
suspended below the orifice by posts, not shown. The deflector
blocks gases from flowing in a straight path from the gas inlet 30
to the directly adjacent holes 22 in the center of the gas
distribution plate, thereby helping to equalize the respective gas
flow rates through the center and periphery of the gas distribution
plate.
Vacuum Seal When Inlet Manifold Side Wall Not Exposed to
Atmosphere
In the embodiments shown in FIGS. 1-11, the upper surface of the
top wall or back wall 28 is the only component of the gas inlet
manifold that is exposed to the ambient atmospheric pressure, hence
the back wall is the only component of the gas inlet manifold that
requires a vacuum seal. Specifically, a vacuum seal between the
chamber interior and the ambient atmosphere outside the chamber is
provided by a first vacuum sealing material 45 between the inlet
manifold back wall 28 and the dielectric spacer 34, and by a second
vacuum sealing material 46 between the dielectric 34 and a surface
of the chamber wall. In the illustrated embodiments, the latter
surface is the surface of the chamber lid 18 on which the
dielectric rests. Because the illustrated embodiments include a
removable chamber lid 18, an additional vacuum sealing material 48
is required between the lid and the chamber side wall 10. Sealing
materials 45, 46 and 48 preferably are O-rings.
In this embodiment, a gas tight seal is not required between the
inlet manifold back wall 28 and the upper flange 26 of the flexible
side walls 24. The only consequence of a gas leak at this junction
would be that a small amount of process gas would enter the chamber
interior through the leak rather than through the orifices 22 in
the gas distribution plate 20. Consequently, in the illustrated
preferred embodiment there is no O-ring between the back wall 28
and the upper flange 26 of the flexible side wall. The upper flange
26 is simply bolted to the back wall 28 by a plurality of bolts 72
inserted in threaded holes spaced around the rim of the back wall.
(See FIG. 4.) Preferably, the bolts 72 clamp the upper flange
between the back wall and a reinforcing bar 27 that is thicker and
more rigid than the upper flange.
In typical operation of the chamber in which the gas distribution
plate or diffuser 20 is to be connected to an RF power supply as
described above, a reliable, low impedance, connection between the
RF power supply and the diffuser is important to maintain a stable
plasma. Because the inlet manifold side walls 24 are metal, they
can provide good RF electrical contact between the gas distribution
plate 20 and the inlet manifold back wall 28. Therefore, the
electrical cable that connects the gas distribution plate to the RF
power supply can be attached directly to the outer surface of the
back wall rather than to the distribution plate. Attaching the RF
cable directly to the gas distribution plate would be undesirable
because it would expose the RF connector to the potentially
corrosive process gas mixture. The bolts 72 help ensure good RF
electrical contact between the inlet manifold back wall 28 and the
upper flange 26 of the flexible side wall 24 of the inlet manifold.
A good RF electrical contact between the lower flange 54 of the
side wall 24 and the diffuser 20 is achieved by the weight of the
diffuser maintaining pressure between the lower flange 54 and a
circumferential groove 21 in the sides of the diffuser. In the FIG.
4 embodiment, weld beads 56 provide additional electrical contact
between the lower flange and the diffuser.
Vacuum Seal When Reinforcing Flange of Side Wall is Exposed to
Atmosphere
In an alternative embodiment shown in FIGS. 12-14, the reinforcing
bar 27 is replaced by an outer reinforcing flange 70 whose
perimeter is exposed to the external ambient atmosphere. This
contrasts with the embodiments of FIGS. 1-11 in which the entire
suspension 24, including the upper flange 26, is completely
enclosed by the perimeter of the top wall or back wall 28 of the
gas inlet manifold. Consequently, in the embodiment of FIGS. 12-14,
the reinforcing flange 70 of the side wall must contribute to the
vacuum seal between the chamber interior and the external ambient
atmosphere, which requires one more O-ring than the previous
embodiments.
As in the previous embodiments, two O-rings 45, 46 or other sealing
material are required on either side of the dielectric spacer 34,
i.e., a first O-ring 45 between the dielectric and the reinforcing
flange 70 of the flexible side wall 24, and a second O-ring 46
between the dielectric and the lid 18. Unlike the previous
embodiments, the present embodiment additionally requires a third
O-ring 47 or other sealing material between the reinforcing flange
70 and the back wall 28 of the inlet manifold.
In order to effect a vacuum seal between the outer reinforcing
flange 70 and the back wall 28 of the gas inlet manifold, the
portion of the reinforcing flange 70 in contact with the third
O-ring 47 must be continuous and uninterrupted around the complete
circle of the O-ring (see FIG. 14), in contrast with the previous
embodiments in which the upper flange 26 did not extend around any
of the four corners of the gas inlet manifold.
There is no need for the flexible side wall or suspension 24 to be
continuous and uninterrupted, since it is not part of the vacuum
seal between the chamber interior and the external ambient
atmosphere. Therefore, it can be four distinct segments as in the
previous embodiments.
A plurality of bolts 72 spaced around the rim of the inlet manifold
back wall 28 attach the reinforcing flange 70 of the suspension 24
to the back wall.
The outer reinforcing flange 70 preferably is shaped as a
rectangular frame with an open center. It can be fabricated by
cutting away or stamping the open center from a rectangular plate.
The outer reinforcing flange 70 of this embodiment replaces the
four reinforcing bars 27 of the previous embodiments. The
reinforcing flange 70 preferably should have a smooth, flat upper
surface abutting the inlet manifold back wall 28. To prevent the
upper flange 26 of the suspension 24 from projecting above the
plane of this upper surface, the upper flange 26 preferably is
attached (e.g., by weld 57) to the reinforcing flange 70 at a shelf
recessed below the upper surface of the reinforcing flange.
As in the previously discussed embodiments of FIGS. 1-11, in our
preferred embodiment of FIGS. 12-14 we prefer to connect the RF
cable directly to the upper surface of the inlet manifold back wall
28. The bolts 72 press the reinforcing flange 70 of the suspension
24 against the back wall 28 and thereby help ensure good RF
electrical contact between the back wall and the suspension. An
important advantage of the present embodiment over the embodiments
of FIGS. 1-11 is that the bolts 72 can be located radially outward
of the O-ring 47. Consequently, the O-ring 47 protects the bolts
72--and, most importantly, the adjacent areas of electrical contact
between the back wall 28 and the reinforcing flange 70 of the
suspension--from exposure to the corrosive process gases and plasma
within the chamber that eventually could degrade the electrical
contact.
Unlike the embodiments of FIGS. 1-11, the embodiment of FIGS. 12-14
leaves the radially outer portion of the reinforcing flange 70
uncovered by the inlet manifold top wall or back wall 28.
Therefore, this embodiment permits the electrical cable 74 from the
RF power supply to be connected directly to the reinforcing flange
70 at an area radially outward of the perimeter of the inlet
manifold back wall 28, as shown in FIG. 15. In this alternative
implementation, because the electrical cable is not connected to
the back wall, there is no need to ensure a low impedance
electrical contact between the side wall 24 and the back wall.
Preferably, in the FIG. 15 embodiment the reinforcing flange 70 is
mechanically mounted to the back wall 28 using the same bolts 72 as
in the embodiment of FIGS. 12-14, although the bolts are not shown
in FIG. 15.
Flexible Suspension to Accommodate Thermal Expansion and
Contraction
A novel and valuable function of the flexible side wall or
suspension 24 of our inlet manifold is that it minimizes mechanical
stresses to the gas distribution plate or diffuser 20 when the
diffuser undergoes thermally induced expansion and contraction.
(The gas distribution plate is referred to as the diffuser for
brevity.) If the diffuser were mounted in the chamber rigidly
rather than by our novel flexible suspension, we expect that
differences in temperature and in thermal expansion coefficients
between the diffuser and the chamber component to which it is
mounted would produce mechanical stress in the diffuser that
eventually would distort or crack the diffuser.
The amount by which the diffuser 20 expands is proportional to both
the size of the diffuser and its temperature. Therefore,
alleviating mechanical stress is most important with the larger
diffusers required to process larger workpieces, such as large flat
panel displays. For reasons described below, it is desirable to
maintain the diffuser at 250.degree. C. to 375.degree. C. during
the operation of a CVD process. We find that at such temperatures
an aluminum diffuser expands by about one percent (1%) in each
dimension. For example, the width of a 30 cm.times.35 cm diffuser
expands by about 3 mm, and the width of a 105 cm.times.125 cm
diffuser expands by about 12 mm. Relative to a fixed reference
point in the center of the diffuser, each edge of the diffuser
expands outward by half this amount (0.5%).
When the width of the diffuser 20 expands in response to its
temperature increase during normal operation of the chamber, it
forces the flexible side wall or suspension 24 to bend outward
(i.e., in a direction roughly perpendicular to the plane of the
side wall) by the amount of the diffuser expansion. The side wall
should be flexible enough to bend by that amount without
substantial force. In particular, the bending force between the
diffuser and the side wall should be low enough to avoid cracking
or distorting the diffuser. More specifically, the bending force
should be low enough to prevent distorting the shape of the
diffuser by more than 0.1 mm=100 microns, more preferably by no
more than 0.025 mm=25 microns, and most preferably by no more than
0.01 mm=10 microns. It is especially important to avoid more than
this amount of distortion of the flatness or contour of the surface
of the diffuser that faces the substrate 14.
We successfully tested two prototypes of the design shown in FIGS.
1-6: one prototype had a 30 cm.times.35 cm diffuser 20 and a 50 mm
tall side wall 24, and the other prototype had a 105 cm.times.125
cm diffuser and a 55 mm tall side wall. In both prototypes the side
wall was sheet aluminum having a thickness of 1 mm. A greater
thickness would be less desirable because it would reduce both the
flexibility and the thermal resistance of the side wall.
Nevertheless, we contemplate that the side wall sheet in the
invention can be as much as 2 mm or 3 mm thick.
Although it is simplest to construct the flexible side wall or
suspension 24 entirely of flexible sheet aluminum so that the side
wall is flexible along its entire height, this is not required. It
suffices for the suspension to include at least one flexible
portion somewhere between the upper portion 26 and the lower end
54.
Design parameters that reduce the bending force are: (1) selecting
a more flexible material for the flexible portion of the
suspension; (2) decreasing the thickness of the flexible portion;
and (3) increasing the length (i.e., height) of the flexible
portion. By length or height we mean the dimension of the flexible
portion of the side wall along the direction perpendicular to the
plane of the diffuser.
As stated above, in response to heating during operation of the
chamber, our 105 cm.times.125 cm diffuser expanded in width by one
percent or 12 mm. Therefore, each of the four side walls was
laterally deflected by half this amount, which is 6 mm. The angle
at which each side wall bends is the lateral deflection of the side
wall divided by the height of the side wall, which in this example
is 6 mm/55 mm=0.11 radians=6.3 degrees. Therefore, in our example,
the side wall or suspension 24 should be flexible enough (i.e.,
sufficiently thin and long) to bend at least 6.3 degrees without
exerting substantial force on the diffuser. As stated above, such
bending force preferably should not distort the shape of the
diffuser by more than 10 or 25 microns.
In the illustrated preferred embodiment, the substrate 14 and the
diffuser 20 are rectangular. Although the flexible side wall 24 can
be a single, unbroken annulus with a rectangular cross section, an
unbroken design is not preferred because thermally induced
mechanical expansion and contraction of the diffuser would produce
excessive stress at the corners of the side wall 24. Our preferred
design for avoiding such stress is to divide the flexible side wall
into four segments or facets, one for each side of the rectangular
diffuser, and to provide at each corner a novel expansion joint
that allows only a negligible amount of gas to leak at the
joint.
Specifically, the inlet manifold side wall or suspension 24
preferably consists of four distinct segments of thin, flexible
sheet aluminum respectively located at the four sides of the
rectangular inlet manifold. (See FIGS. 2 and 3.) Each of the four
segments 24 preferably is formed from a flat, rectangular piece of
sheet metal whose upper end is bent 90.degree. to form an outwardly
extending upper flange 26, and whose lower end is bent 90.degree.
to form an inwardly extending lower flange 54. (See FIG. 4.)
Each of the four upper flanges 26 is reinforced by a rigid bar 27,
preferably a 5 mm thick aluminum bar. Each reinforcing bar 27 is
bolted to the underside of the inlet manifold back wall 28, and the
corresponding upper flange 26 is sandwiched between the reinforcing
bar and the back wall, thereby clamping the upper flange to the
back wall.
Each of the four sides of the diffuser 20 has a circumferential
groove 21 that extends across all, or almost all, of the width of
the diffuser. To attach the flexible suspension or inlet manifold
side wall 24 to the diffuser, each of the four suspension segments
or facets 24 preferably has an inwardly extending lower flange 54
that is inserted in the corresponding groove 21 in the diffuser
(FIG. 4).
In the FIG. 4 embodiment, the lower mounting flange 54 and the
diffuser 20 are secured together by one or more weld beads 56. In
tests of this embodiment with the previously described 300
mm.times.350 mm diffuser, we found that flexible suspension
appeared to function successfully in accommodating thermal
expansion and contraction of the diffuser as it heated and cooled
when the plasma within the chamber was turned on and off during
typical plasma process cycles.
We discovered one shortcoming of the embodiment just described in
which the flexible suspension is rigidly attached to the diffuser.
During normal operation of the chamber, the lid 18 remains closed
at all times. The lid is opened only for scheduled maintenance or
to repair an unexpected problem within the chamber. We discovered
that if the chamber lid is opened without allowing the chamber to
cool off, the low thermal mass of the flexible suspension or
manifold side wall 24 causes it to cool off so much more rapidly
than the diffuser, and hence to contract so much more rapidly, that
the resulting mechanical stress can cause the flexible suspension
to crack.
FIGS. 5 and 6 show a preferred design for attaching the flexible
suspension or inlet manifold side wall 24 to the diffuser 20 so as
to permit each segment of the inlet manifold side wall 24 to slide
within the groove 21 in the diffuser. In our tests of this
preferred embodiment, there was no evidence of cracking of the
inlet manifold side wall or diffuser even when the chamber lid was
opened to ambient atmosphere while the chamber was hot.
The key distinction of the design of FIGS. 5 and 6 is that the
lower flange 54 of each segment of the inlet manifold side wall is
permitted to slide within the groove 21 of the diffuser 20, but the
lower flange includes a constraining feature that prevents the
lower flange from sliding completely out of the groove. In our
preferred embodiment, the constraining feature is a set of one or
more holes 80, 81 in the lower flange 54 and an equal number of
pins 82 attached to the diffuser. Each pin protrudes through a
corresponding one of the holes, thereby constraining the lower
flange from sliding in the groove by an amount greater than the
width of each hole.
In our preferred embodiment, each pin 82 is press fitted into a
hole (not shown) in the diffuser. Alternatively, the holes in the
diffuser could be threaded and screws could be substituted for the
pins, but the screws should be longer than the threaded holes so
that the screw heads cannot be tightened enough to prevent the
lower flange 54 from sliding within the groove of the diffuser.
The process for initially attaching the suspension or inlet
manifold side wall 24 to the diffuser 20 is as follows. With all of
the pins removed from the diffuser, one of the inlet manifold side
wall segments is positioned in the corresponding groove 21 of the
diffuser so that its holes 80, 81 are aligned with the
corresponding holes of the diffuser. The pins 82 are then inserted
so as to pass through the holes 80, 81 of the side wall segment and
are press fitted into the holes in the diffuser. At this point, the
first side wall segment is constrained by the pins so that it
cannot be completely removed from the groove of the diffuser. This
assembly process is repeated for each of the other side wall
segments 24.
FIG. 7 shows a less desirable alternative design in which the
diffuser does not employ a circumferential groove, but merely has a
circumferential lip 84 that rests on the lower flange 54 of the
inlet manifold side wall 24. As in the FIG. 5 embodiment, the lower
flange is attached to the diffuser by a plurality of pins or screws
82 press fitted or threaded into the diffuser that engage holes 80,
81 in the lower flange 54, so the features shown in FIG. 6 remain
the same. One disadvantage of the FIG. 7 embodiment relative to the
preceding embodiments is that the diffuser lip 84 resting on the
lower flange 54 does not create as good a seal against leakage of
gas from the inlet manifold. Another disadvantage is that the lower
flange would tend to bend downward, thereby degrading the RF
electrical contact between the lower flange and the diffuser and
possibly causing the lower flange to crack or break.
FIG. 6 shows the lower flange 54 of each of the four segments of
the inner manifold side wall 24. In each flange 54, the three
centermost holes 80 are circular, and the remaining holes 81 are
elongated. Each of the elongated holes 81 has a short axis (minor
axis) and a long axis (major axis) which are mutually
perpendicular. The long axis of each elongated hole 81 is parallel
to the long axis of the groove 21 in the diffuser into which it the
flange 54 inserted. (To more clearly illustrate the shapes of the
holes 80, 81, FIG. 6 exaggerates the size of each hole and the
width of each lower flange 54 relative to the length of each lower
flange.)
Each segment of the inner manifold side wall 24 will be able to
slide along the long axis of the groove 21 of the diffuser by as
much as a "sliding distance" defined as the difference between the
width of the long axis of each elongated hole 81 and the width,
parallel to such long axis, of the pin 82 that is mated with such
hole. In general, the long axis of each elongated hole should be
large enough that this sliding distance is greater than the maximum
expected difference between the expansion of the side wall segment
and the expansion of the diffuser in response to temperature
gradients during operation of the chamber.
If one wanted to dimension the elongated holes to accommodate a
hypothetical worst case scenario of the diffuser undergoing heating
and thermal expansion while the flexible suspension remained cold
with zero expansion, then the sliding distance of each elongated
hole should be the amount by which each side of the diffuser
expands relative to a fixed point in the center of the diffuser.
Using our estimate that, when heated to about 300.degree. C., each
side of the diffuser expands relative to the center by 0.5% of its
total width, then the sliding distance of each elongated hole
should be 0.5% of the width of the diffuser. Hence, the long axis
of each elongated hole should be this amount plus the width or
diameter of the pin.
One prototype of the invention we tested had a 105 cm.times.125 cm
diffuser. As stated above, the maximum thermal expansion of each
side of a diffuser of this size is about 0.5%=6 mm=0.24 inch.
Therefore, to accommodate the worst case of a hot diffuser and a
cold suspension, the sliding distance of each hole should be 0.24
inch, i.e., the long axis of each elongated hole should exceed the
diameter of a pin by 0.24 inch.
Fortunately, this hypothetical worst case scenario will not occur
in practice, since the diffuser cannot be heated or cooled without
heating or cooling the adjoining portion of the suspension. During
operation of the plasma chamber, the diffuser and side wall are
heated and cooled gradually enough that the lower end of the side
wall remains at almost the same temperature as the diffuser. As
stated above, the greatest temperature differential that actually
occurs in practice is when maintenance personnel open the chamber
lid without waiting for the chamber to cool off. Even in that
situation, we estimate that the temperature differential between
the diffuser and the lower end of the side wall sheet 24 will be no
more than 50.degree. C. Therefore, the difference in thermal
expansion between the diffuser and the lower end of the side wall
will be much less than the total thermal expansion of the
diffuser.
To determine the amount of differential expansion that the
elongated holes should accommodate, we tested in a conventional
plasma CVD chamber a prototype of the gas inlet manifold design of
FIGS. 1-6 in which the diffuser width was 105 cm.times.125 cm, each
pin 82 had a diameter of 0.099 inch (i.e., approximately 0.10
inch), and each elongated hole 81 had a short axis of 0.11 inch and
a long axis of 0.19 inch. Therefore, the sliding distance along the
long axis was 0.19 inch-0.10 inch=0.09 inch. The plasma chamber was
operated in several cycles of heating and cooling, and the chamber
lid was opened several times while the chamber was still hot. We
then removed the gas inlet manifold from the chamber and inspected
the lower flange 54 of the side wall. Slight abrasion marks on the
edges of each elongated hole 81 indicated the distance over which
the pin 82 slid within the hole. As expected, the holes farthest
from the center evidenced the greatest sliding distance, but the
observed distance was only about 0.03 to 0.04 inch. This is much
less than the maximum sliding distance of 0.09 inch permitted by
the long axis of the holes. Therefore, the elongated holes appeared
to provide a substantial margin of safety to accommodate two or
three times more differential thermal expansion than what we
actually observed.
Conversely, the observed sliding distance of no more than 0.04
inch=1 mm is less than 0.1% of the width of the diffuser.
Therefore, it should be possible to accommodate differential
thermal expansion using elongated holes whose long axis exceeds the
corresponding width of the pins by at least 0.03 inch or 0.04 inch
or, more generally, by at least 0.1% of the width of the diffuser.
The primary disadvantage of making the long axis greater than
necessary is that a larger hole weakens the lower flange 54 so as
to increase the risk that it will crack.
The short axis of each elongated hole 81 only needs to exceed the
width of the mating pin 82 parallel to this axis by a very slight
amount sufficient to prevent the pin from binding in the hole, so
that the lower flange 54 will be free to slide along the long axis
without binding. This slight difference in dimensions can be
substantially less than the sliding distance along the long axis
that was discussed in the preceding paragraphs. In the illustrated
preferred embodiment, the short axis of each elongated hole 81 is
0.110 inch, which exceeds the 0.099 inch diameter of each pin 82 by
0.011 inch.
The invention would work if all of the holes in the lower flange 54
of the flexible suspension 24 were elongated as just described.
However, there is no need for the entire lower flange to slide in
the groove of the diffuser. Differential thermal expansion and
contraction can be accommodated just as well if the lower flange is
fixed to the diffuser at one point, so that the remainder of the
lower flange is allowed to slide relative to this fixed point as
the lower flange and diffuser expand and contract. Accordingly, in
our preferred embodiment the three holes 80 closest to the center
of each lower flange 54 are circular rather than elongated. Their
diameter of each circular hole 80 is the same as the short axis of
the elongated holes 81, namely, 0.110 inch. Since freedom from
binding is not required for these fixed points, the circular holes
80 could be as small as the diameter of their corresponding pins
82.
Alternatively, the lower flange 54 of each segment of the
suspension 24 could be welded or otherwise affixed to the diffuser
at one point, preferably near the center of the lower flange 54, in
which case the circular central holes 80 and their corresponding
pins 82 could be omitted entirely.
An advantage of minimizing sliding of the lower flange 54 near its
center, such as by using small circular holes 80 or welding as just
described, is that it maintains the lower flange centered relative
to the diffuser. In the prototype chamber, clearances around the
inlet manifold are very tight, so accurate centering is important.
This benefit also could be achieved using only one single circular
hole 80 instead of three on each lower flange. Three circular holes
were used in the preferred embodiment to ensure accurate centering
even if one of the holes is inadvertently damaged.
The holes 80, 81 are spaced apart by 3.2 inches in the preferred
embodiment. However, this spacing between holes is not critical,
and a wide range of spacings would be expected to work well.
In embodiments (such as FIGS. 5 and 8) in which the top flange 26
of the side wall is directly mounted to the inlet manifold top wall
or back wall 28 by bolts 72, it is preferable to avoid stress that
may be caused by differential thermal expansion between the top
flange 26 and the back plate 28. Accordingly, the holes in the top
flange 26 through which the mounting bolts 72 pass should be
fabricated with the same pattern of circular and elongated as the
holes 80, 81 of the lower flange 54.
Corner Seal for Flexible Suspension
Since the preferred embodiment implements the flexible suspension
or inlet manifold side wall 24 as four separate segments or facets,
two adjacent side wall segments will meet near each of the four
corners of the diffuser. A junction or seal between the edges of
adjacent side wall segments 24 should be provided at each corner so
that excessive process gas does not leak from the inlet manifold
into the chamber at the junction. To preserve the benefit of our
flexible inlet manifold side wall in accommodating thermal
expansion of the diffuser, the junction should accommodate flexing
of the inlet manifold side wall as the diffuser expands and
contracts.
FIGS. 2, 3 and 9 show our preferred junction at each of the four
corners of the diffuser. Both ends 60 of each of the four side wall
segments 24 are bent inward at a 45 degree angle so that, at a
given corner, the respective ends of the two adjacent side wall
segments 24 are coplanar. A moderately gas-tight seal between the
adjacent ends 60 is accomplished by a slotted coupler 62, 64
(alternatively called a slotted cover or slotted sealing member)
that slips over the two ends 60. The coupler is fabricated by
welding together two pieces of sheet aluminum along a vertical
center seam, and bending one coupler piece 62 so as to create a
slot between it and the other coupler piece 64. The slotted coupler
is installed by slipping it over the two ends 60 so that the seam
of the coupler is approximately centered in the gap between the two
ends 60, and so that each end 60 fits snugly in a corresponding one
of the two slots of the coupler. The slot is sized to fit around
the end 60 with sufficient snugness so that it permits an amount of
gas leakage from the inlet manifold to the chamber that is no more
than a small fraction of the intended gas flow through the
perforations 22. Nevertheless, the slot is sized large enough to
permit radial movement of the ends 60 as the diffuser expands and
contracts.
FIGS. 10 and 11 show an alternative design for the slotted cover or
coupler consisting of a single, rectangular piece of sheet metal
66. A pair of rectangular notches is cut out as shown in FIG. 10 so
as to leave only a thin bridge 68 between two halves of the coupler
66. The coupler 66 is folded in half at the bridge as shown in FIG.
11. The width W of the bridge 68 is narrow enough to slide between
the two ends 60 of the two inlet manifold side walls that meet at a
corner. The slotted coupler 66 is installed in the same manner as
the previously described coupler 62, 64: by sliding the coupler 66
over the two ends 60. The length L of the bridge 68 determines the
gap between the two halves of the coupler 66 when it is folded as
shown in FIG. 11. This gap should be large enough to permit
movement of the ends 60 as the inlet manifold side wall flexes in
response to expansion and contraction of the diffuser, but it
should be small enough so that the two halves of the slotted
coupler 66 fit snugly around the ends 60 so as to minimize gas
leakage as described in the preceding paragraph.
Our preferred embodiment additionally includes in each of the four
corners of the gas inlet manifold a corner support post 58 having a
triangular cross section as shown in FIGS. 2, 3, 8 and 9. The
corner support post preferably is bolted to the diffuser 20 as
shown in FIGS. 8 and 9, although alternatively it can be bolted to
the back wall 28 of the inlet manifold. The corner support post
should be spaced outward from the slotted coupler or seal 62, 64 so
as to not interfere with movement of the slotted coupler as the
diffuser expands and contracts.
The four corner support posts 58 perform two functions. The first
function is to impede leakage of gas through the corners of the gas
inlet manifold. This function is accomplished by the lips or wings
59 of the corner post. Each lip or wing 59 is a lateral extension
of the corner post that extends across the interface between the
adjacent slotted coupler 62-66 and the adjacent segment of the
inlet manifold side wall 24 so as to overlap a that side wall
segment 24 by a length sufficient to provide substantial impedance
to gas leakage through the interface. Increasing the length of the
overlap beneficially increases the impedance. In the preferred
embodiment, an overlap of 0.28 inch provided sufficient impedance
to leakage. We expect an overlap of 0.1 inch or greater would
suffice. Although the fabrication method is not important to its
operation, we fabricated each corner post, including the wings, as
a unitary piece by machining a block of aluminum.
To prevent the corner support posts 58 from obstructing relative
motion between the flexible suspension or side wall 24 and the
diffuser 20, each corner support post should be slightly shorter
than the height of the side wall, and should be spaced radially
outward from the adjacent slotted coupler 62-66 by a gap sufficient
to prevent them from abutting when the flexible side wall expands
relative to the diffuser to the maximum expected extent. Likewise,
each lip or wing 59 should be spaced radially outward from the
adjacent segment of the side wall 24 by a gap sufficient to prevent
them from abutting. In the preferred embodiment, both gaps were
about 0.010 to 0.015 inch, and each corner post was about 0.005 to
0.010 inch shorter than the side wall.
The second function of the four corner support posts 58 is relevant
only to maintenance, not operation, of the plasma chamber. This
second function is to prevent the thin side walls 24 from
collapsing when the gas inlet manifold assembly 20-32 is stored
outside the plasma chamber, for example when the manifold assembly
is stored as a spare part, or when it is removed from the plasma
chamber to permit maintenance of the chamber.
Alternatively, the wings 59 can be omitted from the four corner
support posts 58, because the gas leakage at the corners of the
inlet manifold 24 may be minimal enough without the wings.
Furthermore, if convenience of storage and maintenance as described
in the preceding paragraph is not important, the corner posts can
be entirely omitted.
In an alternative design shown in FIG. 16, the four corner covers
or couplers 60-66 and the four corner support posts 58 can be
omitted simply by extending each of the four segments of the
flexible side walls 24 so that they abut at the four corners of the
diffuser. This simplified design may produce more leakage of
process gas at the corners, but in many applications the amount of
leakage may be so small as to not significantly affect the plasma
process being performed on the workpiece.
In a chamber intended to process a circular workpiece 14 such as a
silicon wafer, the diffuser 20 preferably should be circular in
cross section, rather than rectangular as in the preceding
examples. In that case, the flexible suspension or side wall 24 of
the gas inlet manifold could be a single, unbroken piece having an
annular shape. Alternatively, the flexibility of the suspension
could be increased by dividing it into any number of axially
extending segments separated by small axially extending gaps,
similar to the four segments of the rectangular side wall in the
previously discussed embodiments.
While thermal expansion of the diffuser is not a severe problem in
the chambers most commonly used today for processing 200 mm
diameter silicon wafers, thermal expansion will become more
significant as the industry moves to larger diameter wafers, and
hence larger diameter diffusers. Therefore, this is an important
prospective application of the invention.
Thermal Isolation of Gas Distribution Plate
In many semiconductor fabrication processes commonly performed in a
plasma chamber, it is necessary to maintain the substrate 14 at an
elevated temperature. Generally this is accomplished by an electric
heater mounted within the substrate support pedestal 12. The
temperature must be spatially uniform across the entire exposed
(front) surface of the substrate in order to achieve good spatial
uniformity of the fabrication process being performed on the
substrate.
When the substrate has low thermal conductivity, as is true for the
glass substrates used for fabricating flat panel displays, spatial
uniformity of substrate surface temperate is harder to achieve.
Typically there is a 50.degree. C. to 75.degree. C. temperature
drop from the pedestal to the front surface of the substrate.
Consequently, the substrate surface temperature is not determined
solely by the pedestal temperature, but is strongly influenced by
the temperatures of nearby chamber components.
In typical plasma chambers, the diffuser or gas distribution plate
20 is by far the chamber component closest to the substrate surface
(other than the pedestal), so it has by far the greatest effect on
the substrate temperature. Attaining high spatial uniformity of the
temperature of the diffuser is important to attain high spatial
uniformity of the substrate surface temperature.
The temperature of the diffuser is determined by the balance
between: (a) heat transferred to the diffuser from the plasma and
black body radiation from the heated substrate, and (b) heat
conducted from the diffuser to the chamber wall 10. In conventional
designs, the diffuser commonly is 100.degree. C. cooler at its
perimeter than its center because the perimeter of the gas
distribution plate is bolted directly to a chamber lid or side wall
that has high thermal mass and high thermal conductivity, so that
the lid or side wall functions as a heat sink drawing heat away
from the perimeter of the distribution plate. The relatively cool
perimeter of the diffuser reduces the temperature of the perimeter
of the substrate surface, thereby degrading the spacial uniformity
of the temperature of the substrate surface.
In contrast, our novel gas inlet manifold can thermally isolate the
gas distribution plate by providing thermal resistance between the
gas distribution plate and the other chamber components to which it
is mounted, such as the lid 18 and chamber wall 10. One advantage
of this thermal isolation is that it reduces heat loss from the
perimeter of the diffuser, and thereby reduces the temperature
differential between the center and perimeter of the diffuser.
Another advantage of the thermal isolation afforded by the
invention is that it enables the diffuser or gas distribution plate
20 to operate at a higher temperature than conventional designs. A
higher temperature diffuser reduces heat loss from the substrate,
thereby reducing the temperature difference between the substrate
surface and the substrate support pedestal. Consequently, for a
given pedestal temperature the semiconductor fabrication process
can be performed at a higher substrate surface temperature, or,
conversely, for a given substrate surface temperature required by a
process, the pedestal can be operated at a lower temperature, which
can extend the life of the pedestal.
Also, if it is desired to use a conventional in situ plasma process
for cleaning residue from the interior of the chamber, the cleaning
of the gas distribution plate is accelerated if the temperature of
the gas distribution plate is elevated.
To achieve the desired thermal isolation of the gas distribution
plate 20, our inlet manifold side wall 24 (or a portion thereof) is
sufficiently thin, and has sufficient length or height, so that the
thermal resistance of the side wall 24 (or such portion) is large
enough to provide a substantial temperature difference between the
gas distribution plate and the chamber components to which it is
mounted, i.e., the inlet manifold top wall or back wall 28, the
chamber lid 18, the chamber side wall 10, and the O-rings 45-47. By
length or height we mean a dimension along the direction
perpendicular to the plane of the gas distribution plate. In the
successfully tested embodiment of FIG. 1, the inlet manifold side
wall is sheet aluminum having a thickness of 1 mm and a height of 5
cm.
Our preferred temperature for the gas distribution plate 20 while
performing a plasma CVD process is at least 200.degree. C.,
preferably 250.degree. C. to 400.degree. C., and most preferably
300.degree. C. to 325.degree. C. Our inlet manifold side wall 24
has sufficient thermal resistance to allow the gas distribution
plate to reach such temperatures while the outer chamber components
do not exceed 100.degree. C. to 140.degree. C. The chamber wall 10,
lid 18, and inlet manifold top wall or back wall 28 can be
considered to function as heat sinks to maintain the O-rings 45-48
at a sufficiently low temperature.
If the temperature is 300.degree. C. at the gas distribution plate
20 during plasma processing and is 140.degree. C. at the inlet
manifold back wall 28 and O-rings 45-48, then the temperature
differential across the inlet manifold side wall 24 is about
160.degree. C. Our invention contemplates that the side wall
thickness and height preferably should be sufficiently small and
large, respectively, so that such temperature differential is at
least 100.degree. C. after the chamber components reach their
normal operating temperatures during plasma processing.
We compared a plasma chamber using the suspended inlet manifold
design of FIGS. 1-11 with an otherwise identical conventional
chamber in which the diffuser or gas distribution plate 20 is
bolted directly to the inlet manifold top wall or back wall 28. In
both chambers an electrical heater within the substrate support
pedestal maintained the pedestal at 400.degree. C. The top wall or
back wall 28 of the inlet manifold, the chamber lid 18, and the
chamber walls 10 were cooled by water maintained at 85.degree. C.
In the conventional chamber, the diffuser temperature ranged from
250.degree. C. to 150.degree. C. at the center and perimeter,
respectively, a 100.degree. C. spatial variation. In the chamber
employing the suspended inlet manifold according to our invention,
the diffuser temperature was 325.degree. C. and 315.degree. C. at
the center and perimeter, respectively, a spatial variation of only
10.degree. C. Therefore, the invention improved the spatial
uniformity of the diffuser temperature by a factor of ten.
(Although we achieved spatial variation of only 10.degree. C., a
suspended inlet manifold would be within the scope of the invention
if its thermal impedance is sufficient to achieve a spatial
variation no greater than 50.degree. C., or preferably no greater
than 20.degree. C.)
Furthermore, the surface temperature at the center of the substrate
surface was 70.degree. C. cooler than the heated pedestal in the
conventional chamber, but only 25.degree. C. cooler than the
pedestal in the chamber using our invention. Therefore, the
invention achieved a substrate surface temperature 45.degree. C.
higher for a given pedestal temperature, or, conversely, would
allow the pedestal to be operated at a temperature 45.degree. C.
cooler to achieve a given substrate surface temperature.
(Although we achieved a temperature differential of only 25.degree.
C. between the heated pedestal and the center of the substrate
surface, a suspended inlet manifold would be within the scope of
the invention if its thermal impedance is sufficient to achieve
such a differential no greater than 50.degree. C., or preferably no
greater than 35.degree. C.)
The thermal isolation between the diffuser and the back wall of the
gas inlet manifold cannot be decreased completely to zero by
further increasing the thermal resistance of the side wall. In
addition to heat conduction through the side wall, heat will be
transferred by radiation from the diffuser to the back wall. If the
thermal resistance of the side wall is high enough that the heat
transfer by conduction is much less than the heat transfer by
radiation, any further increase in the thermal resistance will
provide little benefit because the heat transfer by radiation will
dominate.
To ensure a reliable vacuum seal between the chamber interior and
the external atmosphere, it is important to protect the O-rings
45-48 from excessive temperature. Low cost O-rings (e.g., composed
of Viton elastomer) typically are rated by their manufacturers at
250.degree. C. or less, and some experts believe such O-rings
should be maintained at or below 100.degree. C. to maximize their
reliability.
The O-rings 46 and 48 directly contact the chamber lid 18, and
O-ring 47 directly contacts the back wall 28 of the gas inlet
manifold, hence the temperatures of these O-rings are expected to
be about the same as the respective temperatures of the lid and
back wall. In the first embodiment, the O-ring 45 directly contacts
the back wall, whereas in the second embodiment (FIGS. 12-14) the
O-ring 45 directly contacts the reinforcing flange 70 of the
suspension 24. Because the reinforcing flange preferably is mounted
in good thermal contact with the back wall, the O-ring 45 in this
embodiment is expected to be only slightly hotter than the other
O-rings.
We find that simple exposure to the ambient atmosphere suffices to
maintain the lid 18 and chamber wall 10 at temperatures of
100.degree. C. to 140.degree. C. The inlet manifold back wall 28
generally is cooler because it has no direct exposure to heat
radiation from the plasma within the chamber. Therefore, we expect
the temperatures of the O-rings 45-48 will not exceed 140.degree.
C. This temperature is low enough that we do not believe any
additional cooling, such as water cooling, is required.
Optionally, however, the chamber side wall 10 can be further cooled
by surrounding it with a water jacket, not shown, through which
cool water can be pumped. Similarly, the cover 16, lid 18, and
inlet manifold back wall 28 can be cooled by pumping the same water
through a sealed water jacket (not shown) mounted on the upper
surface of the inlet manifold back wall 28, below the cover 16.
Such water cooling can prevent the temperatures of the O-rings
45-48 from exceeding 100.degree. C.
Since the top wall or back wall 28 of the gas inlet manifold is RF
powered, a dielectric should be interposed between the water jacket
and the back wall. A thicker dielectric can be selected if it is
desired to increase the temperature differential between the water
jacket and the back wall. This may be useful in applications in
which it is desired to maintain the back wall at a temperature
substantially higher than the temperature of the water, such as a
temperature over 100.degree. C. Maintaining the back wall at such a
high temperature would help elevate the temperature of the gas
distribution plate, which can be advantageous for reasons explained
in the next paragraph.
Thermal Isolation Without Flexible Suspension
The preceding section of this patent specification describes the
benefits of thermal isolation between the diffuser or gas
distribution plate 20 and the chamber components to which the
diffuser is attached. As stated above, such thermal isolation is
attained if the side wall 24 of the inlet manifold has sufficient
thinness and height to interpose substantial thermal impedance
between the diffuser and the chamber component to which the upper
end of the side wall 24 is mounted.
In addition, the inlet manifold side wall 24 preferably is flexible
in order to avoid stress in the diffuser due to differential
thermal expansion between the diffuser and the side wall, as also
described above. While preferable, such flexibility is not
essential to achieve the thermal isolation benefits of the inlet
manifold side wall. For example, to further increase the thermal
isolation, it may be desirable to fabricate the side wall 24 of a
material having much lower thermal conductivity than the aluminum
used in the previously described embodiments. Some such materials
may be too stiff or brittle to be flexible.
If the side wall 24 is not flexible, then some other means should
be used to avoid mechanical stress in the diffuser due to
differential thermal expansion between the diffuser and the side
wall. One solution, shown in FIG. 17, is to enlarge the holes 80,
81 in the lower flange 54 so as to permit the differential movement
between the diffuser and the side wall that was provided by the
flexibility of the side wall in the preceding embodiments. In
particular, the holes 81 that were elongated in the flexible side
wall embodiments should be circular when the side wall is
non-flexible because the diffuser will thermally expand in both
orthogonal directions in the plane of the diffuser.
The diameter of each circular hole 81 should be at least as great
as the long axis of the corresponding elongated holes in the
preceding embodiments. Specifically, the sliding distance of each
pin 82 in its corresponding hole 81 should be equal to or greater
than the maximum expected difference between the expansion of the
side wall segment and the expansion of the diffuser in response to
temperature gradients during operation of the chamber. The diameter
of the hole 81 should be such sliding distance plus the diameter of
the pin 82.
The holes 80 at the center of each lower flange 54 preferably
should be dimensioned to permit the lower flange to move in a
direction perpendicular to the side wall 24, but to maintain the
centering of the diffuser by preventing movement of the lower
flange in a direction parallel to the long dimension of the lower
flange. This can be accomplished if the holes 80 are elongated,
with their short and long axes respectively parallel and
perpendicular the long dimension of the lower flange, as shown in
FIG. 17. The long axis of each centrally located hole 80 should be
dimensioned according to the same criteria as the diameter of the
circular holes 81 as just discussed.
The short axis of each elongated hole 80 only needs to exceed the
diameter of the mating pin 82 by a slight amount sufficient to
prevent the pin from binding in the hole, so that the lower flange
54 will be free to slide along the long axis without binding. This
slight difference in dimensions can be substantially less than the
sliding distance along the long axis that was discussed in the
preceding paragraphs. For example, if each pin 82 has a diameter of
0.1 inch, the short axis of each elongated hole 81 can be 0.11
inch.
* * * * *